A fuel pump mounted in an automobile preferably has a relatively long operating life, a reduced size, and a reduced cost. These features can be mutually contrary to each other. Further, a motor provided with a brush can be used as the drive source of the fuel pump, and the durability of the brush is important for increasing operating life and decreasing costs. However, the brush can be prone to deterioration due to electric discharge and wear between the brush and a commutator.
Also, in order to reduce the size of the fuel pump, the material and structure of the brush have been altered to account for the reduced area of the brush. However, these different brush materials and brush structures can increase costs, which is undesirable.
Furthermore, to satisfy a recent demand for low emission and low fuel consumption, idle stop system and hybrid electric vehicles have been employed in increasing numbers. However, for the idle stop system and the hybrid electric vehicle, since there is an increase in the number of times that an engine (internal combustion engine) is automatically stopped and started, the fuel pump is stopped and started an increased number of times. Further, in the hybrid electric vehicle, a power supply (battery) for driving the fuel pump is different from a power supply (battery) for starting the engine, so there are circumstances that the drive voltage at the time of starting the fuel pump is not reduced by the driving of the starter (cranking of the engine) but becomes higher than ever before.
The electric discharge between the brush and the commutator, which causes the brush of the fuel pump to deteriorate, tends to easily develop due to a rush current at the time of startup, and as a drive voltage at the time of startup increases, a rush current increases, and hence the electric discharge tends to easily develop. Thus, when the number of times that the fuel pump is started and the drive voltage at the time of startup increases, stress applied to the brush by the rush current is increased by a corresponding amount to reduce the durability of the fuel pump. Hence some countermeasures are necessary.
Several proposals have been made for enhancing the durability of the fuel pump. For example, JP-B 60-37313 discloses that when a fuel injection quantity (injection pulse width) during the operation of an engine becomes at most a predetermined value, the drive voltage of the fuel pump is reduced (see pages 1 and 2, etc.). Moreover, JP-B 61-1621 discloses that when an engine is rotated at a low speed and under a low load, the drive voltage of the fuel pump is reduced (see page 1, etc.).
The technologies disclosed in these patent documents is based on the concept that the drive voltage of a fuel pump is reduced in an operating range in which a required fuel quantity is small to reduce a drive current to thereby achieve an elongated life. However, it is desirable to further extend the operating life of the fuel pump employing these technologies.
In other words, as described above, the drive current of the fuel pump is greatly increased because of the rush current at the time of startup, and the stress applied to the brush at the time of startup becomes larger as the drive current becomes larger. Both of the technologies disclosed in the patent document 1, 2 are technology for controlling the drive voltage of the fuel pump after the engine is started (after the fuel pump is started). Therefore, these technologies have little to no effect on the stress applied to the brush due to the rush current at the time of starting the fuel pump.
A brush-less motor can be used as the drive source of the fuel pump in place of a motor provided with a brush as disclosed in the JP-A 03-179158. As such, the durability of the fuel pump can be improved. However, construction of the drive circuit of the brush-less motor is complex and hence increases cost.
The present disclosure is made in consideration of these circumstances. Hence, the object of the present disclosure is to provide in a system using a motor provided with a brush as the drive source of the fuel pump such a drive control device of a fuel pump as can reduce stress applied to a brush by a rush current at the time of starting a fuel pump and can balance the mutually contradictory features of elongated life, reduced size, and reduced cost of the fuel pump.